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Dropping Charges

Yesterday was the birthday of Robert Millikan (born in 1868), the physicist who first measured the charge of an electron--an experiment repeated every year by physics students around the world. In 1908, physicists were struggling to measure the electron's charge with clouds of water droplets. By placing a charge on the droplets, they could tug the droplets upward against gravity with an electric field. Once the droplets were hovering in midair, their mass and the strength of the electric field would reveal the charge of the electron. But evaporation was foiling the measurements, so Millikan substituted oil droplets. His experiment succeeded, and Millikan proved that the electron was indeed an elementary particle with a fundamental charge. He published his value for the charge of an electron in 1913 and 10 years later received the Nobel Prize in physics.